For photographing with a camera, various problems are caused by lens aberration. Examples of typical chromatic aberration include Seidel's five aberrations, which are monochromatic aberrations. This is five aberrations analyzed by Seidel in Germany and based on a lens sphere and is a collective term of spherical aberration, coma aberration, astigmatism aberration, distortion aberration, and field curvature. Apart from those aberrations, it is known that chromatic aberration can also cause a serious problem. The chromatic aberration results from the fact that the refractive index of light with respect to lens material varies due to its wavelength, and produces false color at an image capture surface.
As typical examples of chromatic aberration, axial chromatic aberration, which causes color blur since a focal-point position on an optical axis varies depending on the wavelength, and magnification chromatic aberration, which causes color shift since the image magnification varies depending on the wavelength, are well known. In general, a phenomenon called “purple fringe (purple tringe)” in English is also an important cause of image deterioration. This is a phenomenon in which false color is generated at an edge portion in an image because of a difference in point spread due to a light wavelength. Even when it is not so visible among typical pixels, the presence of a high-contrast edge portion where white saturation, i.e., a state in which the luminance level is saturated, occurs causes purple false color to be generated around the edge portion, thus forming an unnatural image. In general, false color generated in the neighborhood of a white-saturated portion is called “purple fringe” since a large amount of purple-based color is generated. However, the false color may be any colors, such as green-tined color, depending on a lens and photography conditions. Hereinafter, the “purple fringe” refers to a false-color generation phenomenon that occurs at a high-contrast edge portion where white saturation occurs, regardless of color generated.
As a technology for reducing color aberration, lenses using glass made of special material, such as fluorite, are available. However, since high cost is required to manufacture such lenses, the price is also high. Such lenses are used for some high-class cameras, such as lens-replaceable cameras, but cannot be said to be widely used.
Patent Document 1 describes a method for reducing false color generated due to chromatic aberration by performing image processing. In this processing, processing for suppressing color, i.e., reducing color saturation, is performed on a portion where green-channel high-frequency components are high. As measures for a case in which white saturation occurs, Patent Document 1 also discloses a configuration for performing processing for reducing the color saturation of a white-saturated portion by photographing two images with varied exposure and estimating the original luminance of the white-saturated portion.
However, in the processing described in Patent Document 1, since the false color is reduced by reducing the color saturation, the color saturation of original subject color is also reduced to thereby make it impossible to faithfully reproduce the original color of the subject. As a result, there is a problem in that an image that looks unnatural is output. Additionally, in order to estimate the luminance of a white-saturated portion, photography needs to be performed twice. When hand movement or subject movement occurs during the two photography operations, there is a problem in that it is difficult to obtain a correct result.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. 2003-60983